1. field of the Invention
This invention relates to data-clock separators for magnetic data storage devices, and more specifically to apparatus of that type having a missing clock detection feature used in conjunction with magnetic disc drives.
2. Description of the Prior Art
Data is commonly recorded on magnetic discs in the form of precisely timed flux reversals which, when processed through appropriate read-out circuits, produce a so-called double frequency pulse train in which data and clock information alternate. Data pulses may be present (logic "1") or absent (logic "0") in their assigned time slots, while clock pulses are normally always present as timing references for the data pulses. In magnetic disc drives, one common method for delineating between adjacent sectors on a given recording track is by not recording the normal clock pulse at that point on the track, i.e. by generating a "missing clock". A standard method for checking that clock pulses are not being confused for data pulses, i.e. that the data and clock pulses are in proper phase, is by recording a string of data "0" indications at the beginning of each sector of recorded information for clock identification purposes.
A timing problem arises in magnetic media due to the fact that each of the closely spaced flux variations in the magnetic medium tends to affect the adjacent one. As a result, the pulses of the double-frequency pulse train are shifted in time toward the assigned position of any adjacent absent pulse. Further, the recording and recovery may be performed on different disc storage devices, each having their own inherent speed tolerances, resulting in further clock timing variations.
In decoding a pulse train recovered from a disc, it is therefore necessary to perform the following operations:
1. DETERMINE WHICH PULSES ARE CLOCK AND WHICH ARE DATA;
2. SEPARATE DATA PULSES FROM CLOCK PULSES AND CONVEY BOTH TO EXTERNAL CIRCUITS;
3. DETECT MISSING CLOCK PULSES AND PROVIDE AN INDICATION THEREOF TO EXTERNAL CIRCUITS; AND
4. SIMULATE TO THE REST OF THE CIRCUIT THE RECEIPT OF A CLOCK PULSE IF A MISSING CLOCK HAS BEEN DETECTED, TO ENABLE PROPER RECOGNITION OF LATER DATA AND CLOCK PULSES.
Apparatus for carrying out these functions has been known for some time. Typically, this type of apparatus either uses highly complex logic or contains several very precisely timed one-shots, or both. Due to the stringent tolerances which must be observed in order to make the circuit reasonably reliable in spite of shifts and inaccuracies in timing, each precision-timed one-shot must be manually adjusted after assembly by means of a trimming resistor. The resulting technician time is a significant economic detriment in the highly competitive disc drive market, and the multiple adjustments can create further reliability and maintenance problems.
Bit shift problems have been handled in the past by reducing the amount of data recorded during a given time interval space on the disc (i.e. lessening the data packing density) so that timing can be less exact upon read-out. Other solutions have included the use of complex error detection or parity circuits to detect when a bit of data has been missed as a result of bit shift.
Clock rate variations have been rarely corrected for and have been handled by imposing tight tolerances on speed variations and write frequency.